The idea that Paul eventually rejected Mark-John/John-Mark in favor of another companion was introduced by Irenaeus. Most people who write about these matters have never had any real sense of intimacy with his writings beyond the 'catalogue of heresies' in Book One (which they read superficially anyway).
In order to help foster a greater intimacy with the other books of Irenaeus I have placed a link to Chapter Fourteen of Book Three where Irenaeus introduces 'Luke' to his readers. I stress over and over again that we can't continue to read the books that make up his Refutation and Destruction of Knowledge Falsely So Called 'buffet style' (i.e. where we pick a little of this and a little of that to help foster inherited prejudices).
As I have argued in a previous post, Book Three is a systematic refutation of the Alexandrian tradition as developed by Cleemnt in the Letter to Theodore. Did Irenaeus know the contents of to Theodore? I don't know. But he certainly was aware of arguments that appear in the Mar Saba document.
Irenaeus begins [AH iii.1] attacking Clement's central understanding that after some inferior expression of 'faith' was written in the name of Peter and the apostles, the heretics whom Irenaeus attacks claim a superior testimonial 'according to perfect knowledge.' He then references Clement's understanding of how Mark wrote his gospel in a secret manner which required initiation into a mystical program developed by the Evangelist himself [AH iii.2].
It is important to note that Irenaeus never directly references the role of a disciple named Mark anywhere in the chapter save for a statement in the middle of the book that a variant gospel in the name of the Evangelist was central to at least some of the heretics he was referencing - viz. "Those, again, who separate Jesus from Christ, alleging that Christ remained impassible, but that it was Jesus who suffered, preferring the Gospel by Mark, if they read it with a love of truth, may have their errors rectified." [AH iii.11.7]
I have outlined my argument for where parallels exists between the early material in Book Three and the Letter to Theodore in a previous post. I have also noted it can be argued that lurking behind Irenaeus' assertion of the primacy of the Roman Episcopal See [AH iii.4] is the pre-existent claim of the very heretics Irenaeus is struggling against that the Alexandrian See was even older and directly tied to St. Mark, THE witness of the Passion of Christ, or even 'Theorimos' (i.e. witness of God) as the modern Alexandrians boldly identify him.
I argued in that last post also that Irenaeus' famous expression of the Gospel as a four-faced revelation [AH iii. 9 - 11] can be seen to be both a refutation of the pre-existent Alexandrian understanding and a development from the central symbol of that tradition - viz. the throne of St. Mark adorned with the four hiyyot from the Jewish prophetic writings.
The central point now is that Acts must have been introduced as a kind of 'first attempt' to reconcile the Alexandrian tradition with the rest of the world. Yes, Alexandria is glaringly absent from the entire narrative (save for a mere footnote with the introduction of Apollos). However having Mark-John/John-Mark as the albeit ultimately subordinated 'glue' which held the two churches of Peter and Paul together could have been tolerable for an Alexandrian like Clement IN ITS ORIGINAL FORM.
How so?
Well let's consider 'the timeline' of To Theodore for a moment. If John-Mark/Mark-John was indeed the Alexandrian St. Mark (as all Alexandrians have held since the reception of that Antiochene text) then the subordination of head of the Alexandrian Church in this period (c. 50 - 60 CE) would fit within the very parameters of the chronology of To Theodore.
Clement says that it was only after Peter died a martyr that Mark came to Alexandria and wrote the gospel of Mark. Where formerly he was attached to Peter even as a 'son' he attained full manhood c. 70 CE.
How old was Mark when he came to Alexandria? Well let's rephrase the question - how old did the Alexandrian tradition think Mark was when he was a disciple of Jesus. The Passio Petri Sancti portrays him as a young boy dressed in the leather tunic of the high priesthood.
Let's say he was ten for argument sake in the year that Jesus underwent his Passion.
I date the Passion to 37 CE based on the traditional Alexandrian dating of the resurrection on March 25th. We can slide this date back to 33 CE for those who prefer inherited Western presuppositions about the gospel narrative. The bottom line is that one can immediately see how the Alexandrian tradition could have accommodated itself to the idea of Mark subordinating himself to Peter and Paul in the 50's and 60's when he was 'twenty-something' only to go on to write the 'perfect gospel,' establish himself as the head of the Alexandrian tradition and develop its sacred mysteries only when he had attained full manhood and the status of magister.
Again, I can't help but hear that there is some kind of reference to the concept of Christ as magister in Irenaeus appeal to the heretics to accept the Gospel of John in Book Two:
He therefore passed through every age, becoming an infant for infants, thus sanctifying infants; a child for children, thus sanctifying those who are of this age, being at the same time made to them an example of piety, righteousness, and submission; a youth for youths, becoming an example to youths, and thus sanctifying them for the Lord. So likewise He was an old man for old men, that He might be a perfect magister for all, not merely as respects the setting forth of the truth, but also as regards age, sanctifying at the same time the aged also, and becoming an example to them likewise. Then, at last, He came on to death itself, that He might be "the first-born from the dead, that in all things He might have the pre-eminence," the Prince of life, existing before all, and going before all.
They, however, that they may establish their false opinion regarding that which is written, "to proclaim the acceptable year of the Lord," maintain that He preached for one year only, and then suffered in the twelfth month, [but] they are forgetful to their own disadvantage, destroying His whole work, and robbing Him of that age which is both more necessary and more honourable than any other; that more advanced age, I mean, during which also as a teacher He excelled all others. For how could He have had disciples, if He did not teach? And how could He have taught, unless He had reached the age of a magister? [AH ii.22.4]
The arguments that Irenaeus is attacking in Book Two of his Against All Heresies are clearly those which Clement develops in Book Six Chapter Eleven of his Stromata. This is the section of text where Schaff and other scholars have noted reflect Clement's undeclared 'appropriation' of the doctrine of 'those of Mark.'
As I have already noted the reason why Clement and the Marcosians 'think' that Jesus had a one year ministry is because they 'preferred' the Gospel of Mark where this idea is plainly manifested. Of course, as we already noted, Irenaeus makes absolutely plain that 'those who prefer the Gospel of Mark' also argue that Jesus and Christ were two different individuals - Jesus 'suffers' on the Cross while Christ witnesses the Passion 'impassably.'
We already know who the Alexandrian tradition thinks witnessed the Passion. We have already demonstrated that Severus of Al'Ashmunein identifies this same St. Mark as the Christ of the Alexandrian tradition. Is it really that much of a leap of logic to suppose that Irenaeus is already referencing the pre-existent idea that these 'Marcosians' identified 'Christ' as a magister when he wrote the gospel?
The only difference of course is that Irenaeus argues that Jesus and Christ are one and the same.
Nevertheless, when we examine the puzzling question of how the Acts of the Apostles - a text which utterly ignores the Alexandrian tradition - could have been embraced by this very same tradition, I think the Letter to Theodore offers us some assistance.
Clement and the contemporary Alexandrian Church must have understood that the Alexandrian tradition itself was only created after the composition of the Gospel of Mark (which as almost all modern studies conclude was written 'after' the traditional date of Peter's martyrdom).
If then, the mystery religion of Alexandria only developed AFTER this date (to Theodore indicates that Mark's status as 'mystagogue' develops from the composition of the perfect gospel) then all what came before it represent necessarily something 'less than perfect.' As such - when you really think about it - it would be perfectly acceptable to the Alexandrians to accept this 'foreign text' which said that BEFORE Mark attained perfection he hung around Peter, Paul and the rest of the Church - until finally attaining the perfect age of a magister.
The bottom line here is that even if the Letter to Theodore were somehow proved to be a forgery, the logic for how the Alexandrians learned to accept the Acts of the Apostles would necessarily parallel its central historical understanding.
As such when the Mar Saba document says that:
during Peter's stay in Rome he [Mark] wrote an account of the Lord's doings, not, however, declaring all of them, nor yet hinting at the secret ones, but selecting what he thought most useful for increasing the faith of those who were being instructed. But when Peter died a martyr, Mark came over to Alexandria, bringing both his own notes and those of Peter ... he composed a more spiritual Gospel for the use of those who were being perfected ... [and] lead the hearers into the innermost sanctuary of that truth hidden by seven veils. Thus, in sum, he prepared matters, neither grudgingly nor incautiously, in my opinion, and, dying, he left his composition to the church in Alexandria, where it even yet is most carefully guarded, being read only to those who are being initiated into the great mysteries.[To Theod. I.15 - 28]
It is impossible to argue that this understanding is something 'bizarre' or 'unheard of' as some who promote the forgery hypothesis are wont to do. They just haven't taken the time to consider what one would expect from an Alexandrian document in this period.
As I noted many times at this blog, the real mystery is how and why Alexandrians would ever have accepted a text like the Acts of the Apostles, which basically ignores their own tradition. It is akin to how the Jews ever learned to accept the Torah when the city of Jerusalem is never explicitly referenced anywhere in the narrative.
The Acts of the Apostles is a document which promotes the sacredness of Antioch. Given that we know that Clement of Alexandria used and accepted Acts (albeit as many have noted as a document of subordinate value to the gospels) the only way that he could have endured its claims about another city is if (a) he lived in an age where Antioch had already been eclipsed by Rome and (b) the idea that Mark was 'just' a young lad in the period described by this narrative and the superior revelation of the superior Episcopal See of Alexandria was understood to manifest itself when he attained the age of a magister.
In summa - the Mar Saba text isn't strange. Those promoting these ideas haven't bothered to establish any intimacy with the Alexandrian tradition.